1. Field of the Invention
This invention relates to gas lubricated bearings associated with the rotating shafts of gas turbines and turbocompressors and, more particularly, to arrangements for protecting such bearings from excessive heat from the associated machinery.
2. Description of the Prior Art
In gas turbine engines, or any other machinery in which high temperatures rotors are situated adjacent high speed bearings, it is essential to maintain the bearings at an acceptably low temperature. This is normally achieved, when the engine is running, by providing the bearings with an adequate supply of lubricating oil from a pump driven by the engine. However, this supply of cooling oil ceases when the engine stops, and the residual heat in the turbine rotor can be sufficient to damage the bearings adjacent the turbine rotor. Such bearings are especially vulnerable to the effect of heat-soak in the case of a gas turbine engine having a regenerative heat-exchanger and a thermally insulated main casing, because the residual heat in the turbine rotor on shutdown is virtually prevented from being dissipated to atmosphere.
Bearings located adjacent to turbine rotors are exposed to temperatures which are sometimes higher than is desirable because of conduction and the radiation of heat from the elevated temperature rotor. This may occur during steady state operation as well as after shutdown due to transfer of heat from the hot turbine wheels through the thermally conducting shaft to the bearings. In either case, the temperature rise occurring in the bearings may be more than can be tolerated by either the materials of which the bearing is constructed or the lubricant.
Gas lubricated bearings are being considered to overcome the problemof lubricant breakdown, especially that which is due to soak-back after shutdown of the machine. Although the use of such gas bearings reduces the magitude of the problem considerably, certain limitations on the maximum temperature of the bearings must be observed. These limitations relate to the materials used in the bearings, primarily the coatings which are provided as anti-friction coatings for startup and shutdown when the rotational speed of the bearing journal is insufficient to maintain the gas film. In addition, the foils or elastic members of the gas bearings may be made from a material which provides the necessary compliance and elasticity at normal operating conditions while having to exhibit certain properties dictated by the specific application, such as compatibility with certain chemicals, corrosion resistance, etc. Because of such requirements, it may not be possible to use materials which are capable of withstanding the higher operating temperatures of conventional equipment.
Typical materials which are used as coatings on gas bearing foils are: Teflon, polyimide or other elastics, plastics, ceramics and, for the compliant bearing foils, beryllium copper, beryllium nickel, stainless steel, etc. It is important that the material limits and restrictions be maintained in order to provide the desired service life of the machinery. If the temperature exceeds acceptable limits, there may be a loss of wearability of the foil coatings or, in the extreme case, evaporation and destruction of the coating. Similarly, the foil material may creep if the temperature is too high, with attendant loss of elasticity and load bearing capacity.
In general, gas bearings usually require relatively large surface areas. From the standpoint of heat dissipation from the bearing, the larger surface area improves the capability to conduct heat through the shaft. Furthermore, the performance of gas bearings is enhanced if the shaft is constructed from a material with higher thermal conductivity, such as low alloy steels and certain copper alloys. Indeed, some prior art apparatus make special provision to improve the heat dissipation capability of bearings. Examples of such in the prior art may be found in O'Neill U.S. Pat. No. 3,845,619 (compacted copper powder in hollow shaft for heat conduction); Laurizio U.S. Pat. No. 4,116,499 (heat conducting spikes in Teflon bearing pad); Burgermeister et al U.S. Pat. No. 4,261,165 (heat conducting element between bearing sleeve and housing); and Baumann et al U.S. Pat. No. 3,149,819 (conducting inserts in shaft). Such arrangements help to dissipate heat that may develop from imperfections of the bearing foil geometry in conjunction with less than optimum gas films during startup or instantaneous overload. However, while such heat conducting capability is desirable from the standpoint of assisting in dissipating heat from the bearing, it may also tend to increase the heat transfer to the bearing from equipment mounted on the shaft, such as a high temperature rotor, for example.
Recognizing the latter problem, various patentees have disclosed attempts to block heat transfer toward the bearings from an associated heat source. Examples of such prior art are found in Judson et al patent 2,938,659 (interruption of the engaging surfaces with grooves and slots, and provision of a baffle); Wood U.S. Pat. No. 2,958,458 (provision of an air space in the direct heat flow path); Leins U.S. Pat. No. 3,106,381 (use of thin metal parts for lower heat conductivity); Korta et al U.S. Pat. No. 4,156,342 (provision of spaces for cooling air paths in the bearing); Webb U.S. Pat. No. 4,198,192 (heat shield shrouds and use of spacers to define dead air spaces); and Schippers et al U.S. Pat. No. 4,364,717 (spacing of bearing housing from turbine housing with means for permitting cooling air flow through the inter-housing gap).
Other patentees, including Schippers et al, have disclosed the use of bleed air taken from the compressor to cool particular components in turbomachinery, such as turbine blades, turbine exhaust, bearings and/or interstitial spaces within the machinery. Examples of such prior art, some of which also incorporate various ones of the features mentioned hereinabove, include Okano et al U.S. Pat. No. 4,376,617; Becker U.S. Pat. No. 4,127,988; Schinnerer et al U.S. Pat. No. 3,740,163; Bobo U.S. Pat. No. 3,356,340; Bill U.S. Pat. No. 3,287,907; Alford U.S. Pat. No. 2,858,101; Batt U.S. Pat. No. 2,680,001; and Davis U.S. Pat. No. 2,758,785. In Bill, although the engine has hollow shafts, the ends of these shafts are closed and the cooling air is directed along the outside of the bearings and then pressurized by a centrifugal pump to introduce it into a region of the jet exhaust where elevated pressure is encountered.
As may be seen from a consideration of the plethora of different approaches to solving the bearing protection problem, no one solution has found universal acceptance. Efforts are still going forward to realize an improved and effective arrangement for protecting gas lubricated bearings, such as are used in association with turbomachinery, from the high operating temperatures of the turbine and from soak-back of residual heat when the turbine shuts down.